I found a good lead on another board ( http://social.technet.microsoft.com/Forums/en/w7itpronetworking/thread/c30f6649-a0d9-4f5e-8671-904a8f5469bb ). Since the problem had a lot of leads which didn't work, I thought I'd cross-post a solution that worked for me.
Specifically, my problem was that file transfers FROM Windows 7 to XP were slow, measured by seeing network utilization in the Task Manager at about 1%. Transfers from XP to Windows 7 typically used 80-99% of the network bandwidth. These results were achieved whether the transfer was "push" or "pull".
What worked for me: I went to Local Area Network properties, Configure, Advanced Tab, and disabled Large Send Offload v2. The advice to disable autotuning, RSS, set Speed & Duplex to a specific value, remove from homegroup, did nothing. Ultimately, the settings which worked on my Dell XPS 8100 Windows 7 Pro 64-bit workstation were as follows:
ARP Offload - Enable
Flow Control - Auto
Interrupt Modulation - Enable
IPv4 Checksum Offload - Rx & Tx Enabled
Large Send Offload (IPv4) - Enable
Large Send Offload v2 (IPv4) - Disable
Large Send Offload v2 (IPv6) - Disable
Network Address - Not present (radio button)
NS Offload - Enable
Priority & VLAN- Priority & VLAN Enabled
Receive Side Scaling - Enable
RSS Queues - RSS 4 Queues
Speed & Duplex - Auto
TCP & UDP Checksum Offload (IPv4) - Rx & Tx Enabled
TCP & UDP Checksum Offload (IPv6) - Rx & Tx Enabled
VLAN ID - 0 Wake Up Capabilities -
Both WOL Speed - Lowest Speed Advertised
Hope this proves helpful to someone else.